G-protein coupled receptors are cell surface receptors that indirectly transduce extracellular signals to downstream effectors, which can be intracellular signaling proteins, enzymes, or channels, and changes in the activity of these effectors then mediate subsequent cellular events. The interaction between the receptor and the downstream effector is mediated by a G-protein, a heterotrimeric protein that binds GTP. G-protein coupled receptors (“GPCRs”) typically have seven transmembrane regions, along with an extracellular domain and a cytoplasmic tail at the C-terminus. These receptors form a large superfamily of related receptors molecules that play a key role in many signaling processes, such as sensory and hormonal signal transduction. For example, a large family of olfactory GPCRs has been identified (see, e.g., Buck & Axel, Cell 65:175–187 (1991)). The further identification of GPCRs is important for understanding the normal process of signal transduction and as well as its involvement in pathologic processes. For example, GPCRs can be used for disease diagnosis as well as for drug discovery. Further identification of novel GPCRs is therefore of great interest.